The promoter region of the interleukin-6 (IL-6) gene has a putative NF-κb-binding site. We found that a fragment of the IL-6 promoter containing the site specifically binds highly purified NF-κB protein and the NF-κB protein in nuclear extracts of phorbol ester-induced Jurkat cells. Mutations of the NF-κB site abolished complex formation with both purified NF-κB and the nuclear extract protein. Transient expression of chloramphenicol acetyltransferase (CAT) plasmids containing the IL-6 promoter revealed very little activity of the promoter in U-937 monocytic cells and in HeLa cells before stimulation. However, stimulation of U-937 and HeLa cells by inducers of NF-κB led to a dramatic increase in CAT activity. Mutations in the NF-κB-binding site abolished inducibility of IL-6 promoter-col constructs in U-937 cells by lipopolysaccharide, tumor necrosis factor a, the double-stranded RNA poly(IC), or phytohemagglutinin and in HeLa cells by tumor necrosis factor a and drastically reduced but did not completely eliminate inducibility in HeLa cells stimulated by double-stranded RNA poly(IC) or phorbol 12-myristate 13-acetate. These results suggest that NF-κB is an important mediator for activation of the IL-6 gene by a variety of IL-6 inducers in both U-937 and HeLa cells and that alternative inducible enhancer elements contribute in a cell-specific manner to IL-6 gene induction. Because NF-κB is involved in the control of a variety of genes activated upon inflammation, NF-κB may play a central role in the inflammatory response to infection and tissue injury.